Apparatus and method for demodulating signal

ABSTRACT

An apparatus for and a method of signal demodulation which reduces a number of calculations required for demodulating an M-bit codeword which has been modulated from an N-bit codeword. Probability data for each bit of the M-bit codeword is tested to determine if the probability data indicates a high reliability of being either a one or a zero. Bits having the high reliability are compared with corresponding bits in each of a plurality of reference codewords and the codewords having bits which match the corresponding bits having the high reliability are used along with the probability data to calculate a probability of each bit of the N-bit codeword. As a number of bits having the high reliability increases, a number of calculations required to calculate the probability of each bit of the N-bit codeword decreases.

BACKGROUND OF THE INVENTION

[0001] This application claims the priority of Korean Patent ApplicationNo. 2002-31065, filed Jun. 3, 2002, in the Korean Intellectual PropertyOffice, the disclosure of which is incorporated herein by reference.

[0002] 1. Field of the Invention

[0003] The present invention relates to an apparatus for and a method ofsignal demodulation, and more particularly, to an apparatus for and amethod of signal demodulation of a run length limit (RLL) code.

[0004] 2. Description of the Related Art

[0005] Generally, in an environment where there is inter symbolinterference (ISI) of data, for example, in a high density opticalrecording medium, considering problems caused by the ISI phenomena, theRLL code is used to modulate data.

[0006]FIGS. 3A and 3B show RLL encoding tables of a (1, 7) code. FIG. 3Ashows a basic encoding table when a code having a code rate of ⅔ isused, and FIG. 3B shows a replacement table for a violation occurringwhen the basic encoding table of FIG. 3A is used.

[0007] The (1, 7) code is a code in which a minimum number of contiguous0's between a 1 and a next 1 in a modulated signal is one and a maximumnumber of contiguous 0's is seven. The code rate of ⅔ indicatesmodulation of a 2-bit data word into a 3-bit codeword.

[0008] However, when the basic encoding table of FIG. 3A is used, aviolation of the rule described above occurs. For example, when inputdata bits are 0000 according to the basic encoding table of FIG. 3A, theencoded channel bits are 101101, and thus, 1 occurs continuously. Inorder to solve the above problem, the replacement table shown in FIG. 3Bis used so that the encoding rule can be observed considering relationsof input data with previous input data and next input data. Accordingly,in order to obtain a 2-bit data word through decoding, a 9-bit codewordis decoded.

[0009] In a conventional method for decoding an RLL code, a channeldetector, for example a viterbi decoder, receives a channel signal,detects codewords and outputs the detected codewords. An RLL decodingunit decodes the codewords into data words using an RLL decoding table.

[0010] Recently, however, concepts of a soft channel detector and a softmodulator have been introduced. The output of the viterbi decoderdescribed above is a codeword itself, and has a value of 1 or −1. Theabove method is referred to as a hard demodulation and another method isreferred to as soft demodulation. In the soft demodulation, a softchannel detector receives a channel signal, and outputs data indicatinga probability value of a codeword. That is, the data output by the softdemodulation method has an analog value, such as 0.8 or −0.8 instead of1 or −1, and the analog value indicates a probability that the codevalue is 1 or −1. A soft demodulator receives data indicating theprobability value of the codeword, and outputs data indicating theprobability of a data word. Then, a decoder such as a turbo decoderfinally decodes the data word.

[0011] A detailed description of soft modulation may be found in anarticle entitled “Near-optimum Decoding of Product Codes” (R.Pyndiah,IEEE Transactions on Communications, vol. 46, No. 8, ppl003-1010).Explanations of a soft channel detector are detailed in articlesentitled “A Comparison of Optimal and Sub-optimal MAP DecodingAlgorithms Operating in the Log Domain” (P. Robertson, E. Villebrun, andP. Hoeher, Communications, 1995, ICC '95 Seattle, ‘Gateway toGlobalization’, 1995 IEEE International Conference on, vol. 2, 1995pp1009-1013 vol. 2) and “A Viterbi Algorithm with Soft Decision Outputsand Its Applications,” (J. Hagenauer and P. Hoeher, Proc. GLOBECOM, '89,pp1680-1786, Nov. 1989). An explanation of turbo decoding requiring softmodulation is detailed in an article entitled “Turbo Decoding with RLLCode for Optical Storage” (E. Yamade, ISOMO1).

[0012] Referring to FIG. 4, a process in which the soft demodulatordescribed above receives R(n), which is data indicating the probabilityvalue of a codeword, and obtains a likelihood ratio (LR), which is avalue indicating the probability of each bit forming a data word, willnow be explained.

[0013] Here, an example in which the code rate is ⅔ and the RLL(1, 7)code is used will be explained. FIG. 4 shows a decoding table of theRLL(1, 7) code. The first line shows data words with a 2-bit length,which are the results of decoding. The remaining lines show codewordsrepresented in an octal notation, each codeword corresponding to a dataword. For example, codeword 040 corresponds to a binary number 000 100000.

[0014] First, in order to obtain an LR, APP(d=1) and APP(d=0) areobtained. APP(d=1) is a value indicating a probability that demodulateddata is 1, and APP(d=0) is a value indicating a probability thatdemodulated data is 0.

[0015] APP(d=1) and APP(d=0) are obtained by equation 1: $\begin{matrix}{{{APP}\left( {d = 1} \right)} = {\sum\limits_{i = 1}^{s\quad 1}{\exp \left\lbrack {\sum\limits_{l = 1}^{L}\quad \left( {C_{l}^{i} - R_{l}} \right)^{2}} \right\rbrack}}} & (1)\end{matrix}$

[0016] In equation 1, since the code rate is ⅔, L is 9. R_(l) is a valueindicating the probability value of a bit which is the l-th bit of 9data bits respectively indicating probability values of 9 code bitsforming one codeword that is input to the soft demodulator. C_(l) ^(i)is a probability value of a bit which is the l-th bit of 9 code bitsthat form an i-th codeword among s1 codewords corresponding to a dataword having 1 as the first bit. Referring to FIG. 4, since the number ofcodewords corresponding to a data word having 1 as the first bit is 40,s1 is 40. $\begin{matrix}{{{APP}\left( {d = 0} \right)} = {\sum\limits_{h = 1}^{s2}{\exp \left\lbrack {\sum\limits_{l = 1}^{L}\quad \left( {C_{l}^{h} - R_{l}} \right)^{2}} \right\rbrack}}} & (2)\end{matrix}$

[0017] In equation 2, L is 9 and R_(l) is the same as in equation 1.C_(l) ^(h) is a probability value of a bit which is the l-th bit of 9code bits that form an h-th codeword among s2 codewords corresponding toa data word having 0 as the first bit. Referring to FIG. 4, since thenumber of codewords corresponding to a data word having 0 as the firstbit is 40, s2 is 40.

[0018] After obtaining APP(d=1) and APP(d=0) according to equations 1and 2, an LR is obtained by equation 3:

LR=APP(D=1)÷APP(d=0)  (3)

[0019] Meanwhile, if computation of the exponent term is complicated inequations 1 and 2, the computation of the exponent part is not carriedout and instead, APP(d=1) and APP(d=0) are obtained by equations 4 and 5and an LR is obtained by equation 6: $\begin{matrix}{{{APP}\left( {d = 1} \right)} = {{Max}\left\lbrack {\left( {\sum\limits_{l = 1}^{L}\quad \left( {C_{l}^{i} - R_{l}} \right)^{2}} \right),\quad \ldots \quad,\left( {\sum\limits_{l = 1}^{L}\quad \left( {C_{l}^{s\quad 1} - R_{l}} \right)^{2}} \right)} \right\rbrack}} & (4)\end{matrix}$

[0020] In equation 4, R_(l) and C_(l) ^(i) are the same as in equation1, and L and s1 are 9 and 40, respectively, as in equation 1. That is,in equation 4, a maximum value among 40 values satisfying$\sum\limits_{l = 1}^{9}\quad \left( {c_{l}^{i} - R_{l}} \right)^{2}$

[0021] is determined as APP(d=1). $\begin{matrix}{{{APP}\left( {d = 0} \right)} = {{Max}\left\lbrack {\left( {\sum\limits_{l = 1}^{L}\quad \left( {C_{l}^{1} - R_{l}} \right)^{2}} \right),\quad \ldots \quad,\left( {\sum\limits_{l = 1}^{L}\quad \left( {C_{l}^{s\quad 2} - R_{l}} \right)^{2}} \right)} \right\rbrack}} & (5)\end{matrix}$

[0022] In equation 5, R_(l) and C_(l) ^(h) are the same as in equation2, and L and s2 are 9 and 40, respectively, as in equation 2. That is,in equation 5, a maximum value among 40 values satisfying$\sum\limits_{l = 1}^{9}\quad \left( {C_{l}^{h} - R_{l}} \right)^{2}$

[0023] is determined as APP(d=0).

[0024] After APP(d=1) and APP(d=0) are obtained according to equations 4and 5, an LR is obtained by equation 6:

LR=APP(d=1)−APP(d=0)  (6)

[0025] However, where an LR is obtained by the method described above,LR should be calculated using all codewords corresponding to a 1-bitdata word. Accordingly, the more complicated a code is, the longer thecomputation time and the greater the complexity.

SUMMARY OF THE INVENTION

[0026] The present invention provides a signal demodulation apparatuswhich, by using data having high reliability among data indicatingprobability values of a codeword, decreases a number of codewords usedin calculating a value indicating the probability of a data word suchthat computation speed increases and complexity decreases.

[0027] The present invention also provides a method of signaldemodulation by which, by using data having a high reliability amongdata indicating probability values of a codeword, a number of codewordsused in calculation of a value indicating a probability of a data wordis reduced such that computation speed increases and complexitydecreases.

[0028] According to an aspect of the present invention, there isprovided a signal demodulation apparatus which demodulates a code thatis modulated from an N-bit data word into an M-bit codeword. The signaldemodulation apparatus comprises a reliability detection unit whichreceives data comprising a plurality of probability data valuesindicating a probability value of the code, and after forming data of apredetermined length L as one unit, compares an absolute value of eachof the probability data values forming the one unit with a predeterminedreference value, and outputs location information and sign informationof each probability data value having an absolute value greater than thepredetermined reference value; a sign comparison unit which receives thelocation information and sign information of the probability data valuesgreater than the predetermined reference value, and using a plurality ofcodewords stored in advance, compares a sign of a code of a codewordcorresponding to the location of the data with the sign information, andoutputs a plurality of codewords in which the signs are the same as thesigns of the probability data values having an absolute value greaterthan the predetermined reference value; and a data word probabilityvalue calculation unit which receives the plurality of codewords outputfrom the sign comparison unit, and data indicating the probability valueof the code and calculates a value indicating a probability of a bit foreach bit forming the data word.

[0029] The reliability detection unit comprises a reference valuestorage unit which stores the predetermined reference value; an inputdata storage unit which receives the data indicating the probabilityvalue of a code, and stores the data in the units of data of thepredetermined length L; and a determining unit which receives each ofthe probability data values from the input data storage unit, receivesthe predetermined reference value from the reference value storage unit,performs the comparison, and outputs the location and sign informationof the probability data values having an absolute value greater than thepredetermined reference value.

[0030] The sign comparison unit comprises a code table storage unitwhich stores a codeword table in which a plurality of codewordscorresponding to a data word are written; and a comparison unit whichreads codewords from the code table storage unit, performs the signcomparison on each read codeword, and outputs the plurality of codewordsin which the signs are the same as the signs of the probability datavalues having an absolute value greater than the predetermined referencevalue.

[0031] According to another aspect of the present invention, there isprovided a method of demodulating a code that is modulated from an N-bitdata word into an M-bit codeword. The method comprises forming dataindicating probability values of the code into a unit having apredetermined length L; comparing each probability value in the unitwith a predetermined reference value; obtaining location information andsign information of each probability value having an absolute valuegreater than the predetermined reference value; comparing the obtainedlocation information and sign information with corresponding locationand sign information of each of a plurality of predetermined codewordscorresponding to a data word; outputting the codewords having locationand sign information which matches the obtained location and signinformation; and calculating a value indicating a probability of a bitfor each bit forming the data word based on the output codewords and theprobability values forming the unit of predetermined length L.

[0032] In an aspect of the invention, the method of demodulating thecode further comprises storing the predetermined reference value; andstoring the data indicating the probability values of the code in unitsof data of a predetermined length L.

[0033] In an aspect of the invention, the method of demodulating thecode further comprises storing a codeword table including the pluralityof codewords corresponding to the data word.

BRIEF DESCRIPTION OF THE DRAWINGS

[0034] The above and/or other features and advantages of the presentinvention will become more apparent by describing in detail preferredembodiments thereof with reference to the attached drawings in which:

[0035]FIG. 1 is a block diagram of an embodiment of a signaldemodulation apparatus according to the present invention;

[0036]FIG. 2 is a flowchart for explaining an embodiment of a signaldemodulation method according to the present invention;

[0037]FIGS. 3A and 3B are embodiments of RLL encoding tables;

[0038]FIG. 4 is an embodiment of a decoding table of an RLL (1, 7) code;and

[0039]FIG. 5 is a schematic diagram for explaining an example in whichcodewords having a same sign as input data having high reliability aredetermined in the signal demodulation apparatus and the method of signaldemodulation according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0040] The present invention will be explained with reference to anexample in which a code rate is ⅔ and an RLL(1, 7) code is used.

[0041] Referring to FIGS. 1, 4, and 5, a structure and operation of asignal demodulation apparatus according to the present invention willnow be explained. FIG. 1 is a block diagram of an embodiment of a signaldemodulation apparatus according to the present invention. The signaldemodulation apparatus comprises a reliability detection unit 110, asign comparison unit 120, and a data word probability value calculationunit 130. The reliability detection unit 110 comprises an input datastorage unit 111, a reference value storage unit 113, and a determiningunit 115. The sign comparison unit 120 comprises a code table storageunit 121 and a comparison unit 123.

[0042] The input data storage unit 111 receives data values R(n), whichare an output of a soft channel detector, for example, a soft viterbidecoder or a maximum a posteriori probability (MAP) decoder, each of thedata values indicating a probability value of a code bit. Then, theinput data storage unit 111 stores every 9 data values as one unit,e.g., R(1), R(2), . . . R(9) are stored as one unit. In order to decodea 2-bit data word, a 9-bit codeword is needed, and in order to apply 9data values to equations 1, 2, 4, and 5, the 9 data values are stored asone unit and are output to the determining unit 115.

[0043] The reference value storage unit 113 stores a predeterminedreference value and outputs the value to the determining unit 115. Thereference value may change according to an external input to thereference value storage unit 113.

[0044] The determining unit 115 receives data R_(k) (k=1, 2, . . . , 9),which is formed by 9 probability value data values R₁, R₂, . . . , R_(g)as one unit, from the input data storage unit 111, and receives apredetermined reference value S from the reference value storage unit113. Then, the determining unit 115 compares each ABS(R_(k)), that is,an absolute value of each data value in a data unit, with thepredetermined reference value S, and outputs information on the locationand sign of R_(k) values that are greater than the predeterminedreference value, to the comparison unit 123. Although 0.8 is used as thepredetermined reference value in explaining the invention, thepredetermined reference value may change. Accordingly, if the value ofR_(k) is equal to or greater than 0.8, the probability that the datavalue is 1 is very high and therefore the data value has highreliability. If the value of R_(k) is equal to or less than −0.8, theprobability that the data value is −1 is also very high and the datavalue also has high reliability.

[0045] The code table storage unit 121 stores a codeword table in whicha plurality of codewords are written as shown in FIG. 4.

[0046] The comparison unit 123 receives information on the locations andsigns of the plurality of R_(k) values that are greater than thepredetermined reference value from the determining unit 115. Thecomparison unit 123 first reads a plurality of codewords correspondingto a data word having 1 as the first bit, from the code table storageunit 121. Referring to FIG. 4, the codewords corresponding to a dataword having 1 as the first bit are codewords in the third and fourthcolumns, and the number of codewords in the third and fourth columns is40. The comparison unit 123 compares the signs of bits of each codewordin the third and fourth columns with the received information on thesigns of the plurality of R_(k) values that are greater than thepredetermined reference value, and by selecting codewords where therespective signs are the same as the respective signs of the receivedR_(k) values, determines S1, which is a set of thus selected codewords.For purposes of determining whether the signs are the same, a bit of acodeword having a binary value of 1 is assumed to have an associatedplus (+) sign and a bit of a codeword having a binary value of 0 isassumed to have a value of −1 and thus an associated minus (−) sign. Forexample, an octal value of 425, written in binary as 100 010 101 may beexpressed as codeword bits having values of (1, −1, −1, −1, 1, −1, 1,−1, 1).

[0047] The comparison unit 123 reads a plurality of codewordscorresponding to a data word having −1 as the first bit. Referring toFIG. 4, the codewords corresponding to a data word having −1 as thefirst bit are codewords in the first and second columns, and a number ofcodewords in the first and second columns is 40. Then, through signcomparison as described above, the comparison unit 123 determines S2,which is a set of a plurality of codewords selected when the signs arethe same.

[0048]FIG. 5 is a diagram showing an example process for determining S2.Assuming that R_(k) is (−0.8, 0, 0, 0.8, 0, 0.7, 0, 0, −0.9) and thereference value is 0.8, the determining unit 115 outputs to thecomparison unit 123 information on locations, that is, k is 1, 4 and 9,and information on signs, that is, −1, +1, and −1, respectively. Thecomparison unit 123 reads 40 codewords in the first and second columnsof the table of FIG. 4, and performs sign comparison as shown in FIG. 5.For R_(k) values whose location is 1 having the sign of −1, thecomparison unit 123 compares the signs of codewords corresponding tolocation 1 and removes codewords having a sign of +1. Since a code bitof a codeword that is 0 actually corresponds to −1, the code bit 0 isselected. For the locations k of 4 and 9, a corresponding signcomparison is performed and matching codewords, (for example, the first,second and fourth codewords, corresponding to octal 040, 050 and 052,indicated in FIG. 5), are selected and output to the data wordprobability value calculation unit 130. If all the codewords shown inFIG. 4 are expanded to binary form, codewords 240, 250, 252, 042, 044,242 and 244 of the Table of FIG. 4 are also indicated as a match to thesample data set (−0.8, 0, 0, 0.8, 0, 0.7, 0, 0, −0.9).

[0049] The data word probability value calculation unit 130 receives S1from the comparison unit 123, e.g., receives R(1), R(2), . . . , R(9),which are the same data as the data input to and stored in the inputdata storage unit 111, and then calculates APP(d=1) according toequation 1 or 4. The data word probability value calculation unit 130receives S2 from the comparison unit 123 and calculates APP(d=0)according to equation 2 or 5. Meanwhile, a value of s1 in equations 1and 4 is, for example, 40, in the related art described above, but thenumber of elements of the set S1 is less than 40 in the embodiment ofthe present invention. Also, the value of s2 in equations 2 and 5 is 40in the related art described above, but a number of elements of the setS2 is also less than 40 in the embodiment of the present invention.Accordingly, a number of computations needed to calculate equations 1through 6 is reduced. The data word probability calculation unit 130calculates APP(d=1) and APP(d=0) and then, using equation 3 or 6,calculates and outputs an LR. The LR, which is the output of the dataword probability value calculation unit 130, is input to a decoder suchas a turbo decoder and a data word is finally decoded.

[0050] Referring to FIGS. 1 and 2, an embodiment of a method of signaldemodulation according to the present invention will now be explained.FIG. 2 is a flowchart for explaining an embodiment of a signaldemodulation method according to the present invention. The method ofsignal demodulation may, for example, be implemented by the signaldemodulation apparatus shown in FIG. 1.

[0051] Data R(n) that indicates a probability value of a code bit isinput in operation 210. After receiving R(n), L data bits R₁, R₂, . . ., R_(L), are stored as one unit in operation 220.

[0052] An absolute value of each probability value, ABS(R_(k)), of dataR_(k) (k=1, 2, . . . , L) that is formed by L probability values R₁, R₂,. . . , R_(L) as one unit is compared with the predetermined referencevalue S in operation 230.

[0053] Location information j of R_(k) values that are greater than thepredetermined reference value S and sign information r_(j) indicating asign of R_(k), are determined in operation 240.

[0054] Codewords corresponding to a data word having 1 as a first bitand codewords corresponding to a data word having −1 as the first bitare selected and read from a code storage table in in operation 250.

[0055] Among codewords corresponding to a data word having 1 as thefirst bit and a data word having −1 as the first bit, the signs of bitsof a codeword corresponding to the location information j of a pluralityof R_(k) values that are greater than the predetermined reference valueS are compared with sign information r_(j). By selecting a codeword whenthe signs are the same, S1, which is formed by codewords selected amonga plurality of codewords corresponding to the data word having 1 as thefirst bit, and S2, which is formed by codewords selected among aplurality of codewords corresponding to the data word having −1 as thefirst bit are determined in operation 260.

[0056] APP(d=1) is calculated according to equation 1 or 4, and APP(d=0)is calculated according to equation 2 or 5. The value of s1 in equations1 and 4 becomes the number of elements of the set S1, and the value ofs2 in equations 2 and 5 becomes the number of elements of the set S2 inoperation 270.

[0057] An LR is calculated using equation 3 or 6 in operation 280.

[0058] The present invention may be embodied in a computer readable codewhich may be embodied in a computer readable recording medium. Thecomputer readable recording medium may include any kind of recordingapparatus on which computer readable data may be stored.

[0059] The computer readable recording media includes storage media suchas magnetic storage media (e.g., ROM's, floppy disks, hard disks, etc.),optically readable media (e.g., CD-ROMs, DVDs, etc.) and carrier waves(e.g., transmissions over the Internet). Also, the computer readablerecording media may be allocated on computer systems which are connectedthrough a network and which store and execute a computer readable codein a distributed mode.

[0060] As described above, by using data having high reliability amongdata indicating probability values of a codeword, the signaldemodulation apparatus and method according to the present inventiondecrease a number of codewords used in calculation of a value indicatingthe probability of a data word such that a computation speed increasesand complexity of the system decreases.

What is claimed is:
 1. A signal demodulation apparatus which demodulatesa code that is modulated from an N-bit data word into an M-bit codeword,comprising: a reliability detection unit which receives data indicatingprobability values of the code, and after forming the probability valuesinto a unit having a predetermined length L, compares an absolute valueof each probability value forming the one unit with a predeterminedreference value, and outputs location information and sign informationof the probability data values forming the one unit that have anabsolute value greater than the predetermined reference value; a signcomparison unit which compares the output location and sign informationwith corresponding location and sign information of respective ones of aplurality of codewords stored in advance and outputs the codewords inwhich the location and sign information of the respective codeword arethe same as the output location and sign information output by thereliability detection unit; and a data word probability valuecalculation unit which calculates a value indicating the probability ofa bit for each bit forming the data word based on the codewords outputby the sign comparison unit and the data indicating the probabilityvalues of the code.
 2. The signal demodulation apparatus of claim 1,wherein the reliability detection unit comprises: a reference valuestorage unit which stores the predetermined reference value; an inputdata storage unit which receives the data indicating the probabilityvalues of the code, and stores the data in units of data of apredetermined length L; and a determining unit which receives each ofthe probability values from the input data storage unit, receives thepredetermined reference value from the reference value storage unit,performs the comparison, and outputs the location and sign informationof the probability values having an absolute value greater than thepredetermined reference value.
 3. The signal demodulation apparatus ofclaim 1, wherein the sign comparison unit comprises: a code tablestorage unit which stores a codeword table in which a plurality ofcodewords corresponding to a data word are written; and a comparisonunit which reads codewords from the code table storage unit, performsthe sign comparison of the output location and sign information withrespective location and sign information of each read codeword, andoutputs a plurality of codewords in which the location and signinformation of the respective codeword are the same as the outputlocation and sign information output by the reliability detection unit.4. The signal demodulation apparatus of claim 1, wherein thepredetermined length L is 3 times a length of the M-bit codeword.
 5. Thesignal demodulation apparatus of claim 2, wherein the predeterminedlength L is 3 times a length of the M-bit codeword.
 6. The signaldemodulation apparatus of claim 3, wherein the sign comparison unitreads codewords corresponding to a data word having 1 as a first bit,from the code table storage unit, performs the sign comparison, andoutputs S1, which is a set of a plurality of codewords in which thesigns are the same, and reads codewords corresponding to a data wordhaving −1 as the first bit, from the code table storage unit, performsthe sign comparison, and outputs S2, which is a set of a plurality ofcodewords in which the signs are the same.
 7. The signal demodulationapparatus of claim 6, wherein the data word probability valuecalculation unit obtains an LR, which is a value indicating aprobability of each bit forming the data word, according to thefollowing equations: $\begin{matrix}{{{APP}\left( {d = 1} \right)} = {\sum\limits_{i = 1}^{s\quad 1}{\exp \left\lbrack {\sum\limits_{l = 1}^{L}\quad \left( {C_{l}^{i} - R_{l}} \right)^{2}} \right\rbrack}}} \\{{{APP}\left( {d = 0} \right)} = {\sum\limits_{h = 1}^{s\quad 2}{\exp \left\lbrack {\sum\limits_{l = 1}^{L}\quad \left( {C_{l}^{h} - R_{l}} \right)^{2}} \right\rbrack}}} \\{{LR} = {{{APP}\left( {d = 1} \right)} \div {{APP}\left( {d = 0} \right)}}}\end{matrix}$

where s1 is a value corresponding to a number of elements of the set S1,s2 is a value corresponding to a number of elements of the set S2, R_(l)is a value indicating a probability value of a bit which is the l-th bitof the probability value data having the unit length L, C_(l) ^(i) is avalue indicating a probability value of a bit which is the l-th bit of Lcode bits forming an i-th codeword among the s1 codewords, and C_(l)^(h) is a value indicating a probability value of a bit which is thel-th bit of L code bits forming an h-th codeword among the s2 codewords.8. The signal demodulation apparatus of claim 6, wherein the data wordprobability value calculation unit obtains an LR, which is a valueindicating a probability of each bit forming the data word, according tothe following equations: $\begin{matrix}{{{APP}\left( {d = 1} \right)} = {{Max}\left\lbrack {\left( {\sum\limits_{l = 1}^{L}\quad \left( {C_{l}^{1} - R_{l}} \right)^{2}} \right),\quad \ldots \quad,\left( {\sum\limits_{l = 1}^{L}\quad \left( {C_{l}^{s\quad 1} - R_{l}} \right)^{2}} \right)} \right\rbrack}} \\{{{APP}\left( {d = 0} \right)} = {{Max}\left\lbrack {\left( {\sum\limits_{l = 1}^{L}\quad \left( {C_{l}^{1} - R_{l}} \right)^{2}} \right),\quad \ldots \quad,\left( {\sum\limits_{l = 1}^{L}\quad \left( {C_{l}^{s\quad 2} - R_{l}} \right)^{2}} \right)} \right\rbrack}} \\{{{LR} = {{{APP}\left( {d = 1} \right)} - {{APP}\left( {d = 0} \right)}}}\quad}\end{matrix}$

where s1 is a value corresponding to a number of elements of the set S1,s2 is a value corresponding to a number of elements of the set S2, R_(l)is a value indicating a probability value of a bit which is the l-th bitof the probability value data having the unit length L, C_(l) ^(k) inthe equation to obtain APP(d=1) is a value indicating a probabilityvalue of a bit which is the l-th bit of L code bits forming a k-thcodeword among the s1 codewords, and C_(l) ^(j) in the equation toobtain APP(d=0) is a value indicating a probability value of a bit whichis the l-th bit of L code bits forming a j-th codeword among the s2codewords.
 9. A method of demodulating a code that is modulated from anN-bit data word into an M-bit codeword, comprising: forming dataindicating probability values of the code into a unit having apredetermined length L; comparing each probability value in the unitwith a predetermined reference value; obtaining location information andsign information of each probability value greater than thepredetermined reference value; comparing the obtained locationinformation and sign information with corresponding location and signinformation of each of a plurality of predetermined codewordscorresponding to a data word; outputting the codewords having locationand sign information which match the obtained location and signinformation; and calculating a value indicating a probability of a bitfor each bit forming the data word based on the output codewords and theprobability values forming the unit of predetermined length L.
 10. Themethod of claim 9, further comprising: storing the predeterminedreference value; and storing the data indicating the probability valuesof the code in units of data of a predetermined length L.
 11. The methodof claim 9, further comprising: storing the predetermined codewords in acodeword table.
 12. The method of claim 9, wherein: the comparing of thelocation information and sign information with corresponding locationand sign information, and the outputting of the stored codeword havinglocation and sign information which matches the obtained location andsign information comprises: selecting first codewords corresponding to adata word having 1 as a first bit from the predetermined codewords,performing the sign comparison on the first codewords, outputting thefirst codewords which match the obtained location and sign informationas a set S1, selecting second codewords corresponding to a data wordhaving −1 as a first bit from among the predetermined codewords,performing the sign comparison on the second codewords, and outputtingthe second codewords which match the obtained location and signinformation as a set S2.
 13. The signal demodulation method of claim 9,wherein the predetermined length L is 3 times a length of the M-bitcodeword.
 14. The signal demodulation method of claim 10, wherein thepredetermined length L is 3 times a length of the M-bit codeword. 15.The signal demodulation method of claim 12, wherein in the calculatingof a value indicating a probability of a bit, an LR that is a valueindicating the probability of each bit forming the data word is obtainedaccording to the following equations: $\begin{matrix}{{{APP}\left( {d = 1} \right)} = {\sum\limits_{i = 1}^{s\quad 1}{\exp \left\lbrack {\sum\limits_{l = 1}^{L}\quad \left( {C_{l}^{i} - R_{l}} \right)^{2}} \right\rbrack}}} \\{{{APP}\left( {d = 0} \right)} = {\sum\limits_{h = 1}^{s\quad 2}{\exp \left\lbrack {\sum\limits_{l = 1}^{L}\quad \left( {C_{l}^{h} - R_{l}} \right)^{2}} \right\rbrack}}} \\{{LR} = {{{APP}\left( {d = 1} \right)} \div {{APP}\left( {d = 0} \right)}}}\end{matrix}$

where s1 is a value corresponding to a number of elements of the set S1,s2 is a value corresponding to a number of elements of the set S2, R_(l)is a value indicating a probability value of a bit which is the l-th bitof the probability value data having the unit length L, C_(l) ^(i) is avalue indicating a probability value of a bit which is the l-th bit of Lcode bits forming an i-th codeword among the s1 codewords, and C_(l)^(h) is a value indicating a probability value of a bit which is thel-th bit of L code bits forming an h-th codeword among the s2 codewords.16. The signal demodulation method of claim 12, wherein in thecalculating of a value indicating a probability of a bit, an LR that isa value indicating the probability of each bit forming the data word isobtained according to the following equations: $\begin{matrix}{{{APP}\left( {d = 1} \right)} = {{Max}\left\lbrack {\left( {\sum\limits_{l = 1}^{L}\quad \left( {C_{l}^{1} - R_{l}} \right)^{2}} \right),\quad \ldots \quad,\left( {\sum\limits_{l = 1}^{L}\quad \left( {C_{l}^{s\quad 1} - R_{l}} \right)^{2}} \right)} \right\rbrack}} \\{{{APP}\left( {d = 0} \right)} = {{Max}\left\lbrack {\left( {\sum\limits_{l = 1}^{L}\quad \left( {C_{l}^{1} - R_{l}} \right)^{2}} \right),\quad \ldots \quad,\left( {\sum\limits_{l = 1}^{L}\quad \left( {C_{l}^{s\quad 2} - R_{l}} \right)^{2}} \right)} \right\rbrack}} \\{{{LR} = {{{APP}\left( {d = 1} \right)} - {{APP}\left( {d = 0} \right)}}}\quad}\end{matrix}$

where s1 is a value corresponding to a number of elements of the set S1,s2 is a value corresponding to a number of elements of the set S2, R_(l)is a value indicating a probability value of a bit which is the l-th bitof the probability value data having the unit length L, C_(l) ^(k) inthe equation to obtain APP(d=1) is a value indicating a probabilityvalue of a bit which is the l-th bit of L code bits forming a k-thcodeword among the s1 codewords, and C_(l) ^(j) in the equation toobtain APP(d=0) is a value indicating a probability value of a bit whichis the l-th bit of L code bits forming a j-th codeword among the s2codewords.
 17. A computer readable medium having embodied thereon acomputer program for a method of demodulating a code that is modulatedfrom an N-bit data word into an M-bit codeword and is transmittedthrough a channel, the computer program comprising: instructions whichenable a computer to form data indicating probability values of the codeinto a unit having a predetermined length L; instructions which enablethe computer to compare each probability value in the unit with apredetermined reference value; instructions which enable the computer toobtain location information and sign information of each probabilityvalue greater than a the predetermined reference value; instructionswhich enable the computer to compare the obtained location and signinformation with corresponding location and sign information of each ofa plurality of predetermined codewords corresponding to a data word;instructions which enable the computer to determine codeword among theplurality of predetermined codeword which have location and signinformation that match the obtained location and sign information; andinstructions which enable a computer to calculate a value indicating aprobability for each bit forming the data word based on the determinedcodeword and the probability values forming the unit of thepredetermined length L.
 18. A method of demodulating a code that ismodulated from an N-bit data word into an M-bit codeword, the methodcomprising: inputting the M-bit codeword wherein each bit of the inputM-bit codeword is expressed as a value indicating a probability that thebit is a one or a zero; assigning a first reference sign to each bitlocation where the respective probability value expresses a highprobability of being a one; assigning a second reference sign to eachbit location where the respective probability value expresses a highprobability of being a zero; determining codewords, from among aplurality of reference M-bit codewords, which have a one at each bitlocation for which a respective first reference sign is assigned andhave a zero at each bit location for which a respective second referencesign is assigned; and calculating a value indicating a probability of abit for each bit forming the N-bit data word based on the determinedreference M-bit codewords and the expressed probability values.
 19. Themethod of claim 18, wherein the high probability is a probability of atleast 0.8.
 20. A method of demodulating a code that is modulated from anN-bit data word into an M-bit codeword, the method comprising: inputtingthe M-bit codeword wherein each bit of the input M-bit codeword isexpressed as a probability of the bit being 1 or 0; determiningrespective bit locations for each bit having a high probability of beingeither 1 or 0; determining codewords, from among a plurality ofreference M-bit codewords, which have a 1 at each bit locationdetermined to have a high probability of being a 1 and have a 0 at eachbit location determined to have a high probability of being a 0; andcalculating a value indicating a probability of a bit for each bitforming the N-bit data word based on the determined reference M-bitcodewords and the expressed input probabilities.
 21. The method of claim20, wherein the high probability is a probability of at least 0.8.